It's main purpose is to get off a Grass Gem Leaf Storm for massive damage, then Trick to get the opponents item and give them nothing. Or you can just Trick right off the bat to give them a Grass Gem. Volt Switch is for momentum and HP Fire is there to get Ferroseed/Ferrothorns out of the way.

Volt Switch is a great move for Rotom-C because it helps it support offensive teams by preserving momentum. You can opt to use Thunderbolt instead, as it helps Rotom-C better take on the likes of SubDD Gyarados. Leaf Storm still does a decent amount of damage to the ever-present Water- and Ground-type Pokemon even without investment. Will-O-Wisp cripples physical attackers that would normally switch into Rotom-C, such as Ferrothorn and Dragonite. Pain Split is a mandatory move on this set because Rotom-C lacks reliable recovery outside of Leftovers, and Pain Split allows it to heal off counters such as Chansey and Blissey. It also works very well with Rotom-C's low HP.

Leaf Storm is the a great Grass-Type move, with that nasty SpAtk drop.

Confuse Ray + Thunder Wave is what my shiny Mow-Rotom has in my X game, and it has saved me in many WiFi battles, it is an annoying combo.

The last slot is really your choice. Thunder if you want power, Thunderbolt if you want accuracy, or Volt Switch to get out of there. Volt Switch is nice combo with Leaf Storm, that way you can get rid of the SpAtk drop when you switch out, but at the cost of power.

You can also replace Thunder Wave or Confuse Ray with Will-O-Wisp if you want to wear down health and cut their attack. You can also give Rotom the move Trick, then give your opponent a choice scarf, or another choice item.

So the idea is to use a few charge beams to get your special-attack higher than it already is. That way, you can use leaf storm, which combined with the grass gem and the high special-attack, deals significant damage. Since charge beam raised your special-attack, leaf storm will bring it back to normal instead of plummeting it. Thief is there to take your opponent's item after you've used the grass gem, and since the ground weakness is taken care of by levitate, hidden power covers against fire and ice types.